It is well known that the sensitivity of the human eye to small variations in shades of gray is intrinsically nonlinear with respect to the optical density of film. According to the literature (see: C. J. Bartleson and E. J. Breneman, "Brightness perception in Complex Fields", J. Opt, Soc. Am., vol. 57, pp. 953-957 (1967). J. L. Mannos and D. J. Sakrison, "The Effects of a Visual Fidelity Criterion on the Encoding of Image", IEEE Trans. Inf. Theory, vol. 20, pp. 525-536 (1974).), the relationship between perceived lightness (S.sub.f) and film density (D.sub.f) was measured to be ##EQU1## where c.sub.1 and c.sub.2 are normalization constants, L.sub.f is the luminance from film, K is the view-box luminance, and p is close to 1/3.
Conventionally, a Cathode Ray Tube Multi-Format Camera (CRT MFC) is calibrated to produce a gray scale on film which matches the gray scale of the monitor (see: R. P. Schwenker, "Film Selection Considerations for Computed Tomography and Ultrasound Video Photography", SPIE, vol. 173, pp. 75-80 (1979); E. R. Ritenour, S. N. Sahu, R. P. Rossi, and T. R. Nelson, "Quantitative Methods for Hard-Copy Device Adjustment", SPIE, vol. 767, pp. 529-535 (1987); H. Shalit, "Method and System in Video Image Hard Copy Reproduction", U.S. Pat. No. 4,939,581, Jul. 3, 1990; L. G. Glasser, A. H. McKinney, C. D. Reilly, and P. D. Schnelle, "Cube-Root Color Coordinate System", J. Opt. Soc. Amer., vol. 48, pp. 736-740, 1958.) However, there is no universal or standard gray scale for the monitor. Usually, the gray scale on the monitor is subjectively set up by the individual radiologist or technician. Therefore, the "look" of the images on film will vary and depend on the setup of the monitor.
Alternatively, a CRT MFC can be calibrated to give a gray scale on film with uniform quantization in density, i.e. the output film density varies linearly with the input code value. From Eq. (1), the perceived lightness will be a nonlinear function of digital image code value. As a result, it would be difficult to see the differences in gray shades in film at high-densities under standard view-box conditions.
The following patents also disclose various video printers, none of which provide a solution to the problem loss of low gray scale and high gray scale detail.
U.S. Pat. No. 4,742,397, issued May 3, 1988, inventors Ferla et al.; PA1 U.S. Pat. No. 4,794,460, issued Dec. 27, 1988, inventor Shiota; PA1 U.S. Pat. No. 4,586,085, issued Apr. 29, 1986, inventor Haendle; PA1 U.S. Pat. No. 4,962,433, issued Oct. 3, 1990, inventor Matsushima; PA1 U.S. Pat. No. 4,709,271, issued Nov. 24, 1987, inventors Yamaguchi et al.; and PA1 U.S. Pat. No. 5,018,085, issued May 21, 1991, inventor Smith.